Results of our studies demonstrate, for the first time, that biological systems are affected by qualitative differences in X-irradiation. The evidence includes differences in the 30 day LD 50 values and differences in the slope of the mortality curves. Differences in dorsal, ventral and two-directional exposures delivered simultaneously at half the dose rate were characterized and related to the relative uniformity of the dose to the bone marrow. The protective effect of anesthesia during irradiation was studied in relation to recovery time and the reduction of oxygen tension in the bone marrow.